The present invention generally relates to the technology for automatically focusing infrared cameras and more particularly to an automatic focusing system for an infrared camera wherein the focusing is achieved by an electronic circuit based upon an image signal produced by the camera, while maintaining a smooth driving operation of an optical system of the camera.
Infrared cameras produce a visual image of a target based upon a detection of infrared radiation from the target. Thus, infrared cameras are used extensively for civil as well as military purposes for measuring surface temperature distribution profiles of objects or detection of a high temperature target.
In such infrared cameras, it is necessary to provide an automatic focusing system such that an arbitrary target in the view field is kept properly focused even when the distance between the target and the camera has been changed. Generally, such an automatic adjustment of focusing is achieved by electronic means by changing a lens position, which is defined as a distance between a lens and the image plane of the optical system in the camera. During such an automatic focusing process, it is desired to change the lens position as smoothly as possible to avoid shock or excessive force to the driving mechanism that is used for driving the lens.
Thus, the present invention is primarily related to the improvement of infrared imaging systems by providing automatic focusing control means. However, the present invention is also useful generally in the sensor fusion technologies wherein various sensing means are integrated.
Conventionally, infrared cameras are provided with a fixed optical system wherein the focal point is fixed at an infinite distance. There are also some infrared cameras equipped with an optical system that allows manual, preset focusing adjustment. There are few infrared cameras, particularly those used in the thermal infrared band, that are equipped with an automatic focusing control system. The reason for this rather primitive state of infrared camera technology may be attributed to the fact that the infrared cameras for thermal infrared radiation require a mechanism for compensating for the temperature variation of the lens which may be caused by the infrared radiation. Provision of such a mechanism makes the structure of cameras substantially more complicated than other types of cameras designed for visible optical radiation.
Although there are a few infrared cameras that are equipped with an automatic focusing mechanism, such a mechanism generally uses an external range finder, such as an ultrasonic range finder, that detects the distance by a sensor. There are no infrared cameras that achieve automatic focusing adjustment based upon an image signal that is obtained by the camera itself.
In the television cameras for visible band images, a CCD array is commonly used, and automatic focusing is achieved based upon the output signal of the CCD array. Thereby, this automatic focusing mechanism generally achieves a focusing adjustment with respect to the object at the central part of the view field. However, it has not been possible to achieve focusing of the optical system with respect to any arbitrary object in the view field.
Thus, conventional infrared cameras, even if equipped with an automatic focusing system, can not achieve the focusing of any arbitrary target in the view field. In order to achieve proper focusing, therefore, it has been necessary to point the camera straight at the target such that the target is held at the center of the view field. However, there are cases where such a control of the camera in relation to the target is not always possible.
In another aspect of the state of the art technology of high sensitivity infrared imaging systems, it should be noted that the two-dimensional array of CCD elements, used conventionally in the visible band imaging systems, cannot be used for the image sensor unit. At present, it is necessary to use a one-dimensional array of infrared detection elements for the image sensor of the infrared camera. Thereby, an optical scanning system is provided which scans the infrared image with respect to the one-dimensional array in the direction perpendicular to the direction in which the one-dimensional sensor array extends. In such a construction, it should be noted that the output signals are produced from the one-dimensional array of infrared sensor elements with a timing that is substantially different from the timing of the output signals produced by the CCD array in the conventional television camera.
Further, in order to implement the automatic focusing adjustment by driving the lens system based upon only the electric signal, it is necessary to establish a control logic for controlling the rotational speed or direction of the motor that drives the lenses. Because of the fact that the hardware for the automatic focusing control system for the infrared cameras does not exist, there is no established logic for controlling such hardware.
In the situation wherein there is a movement of the target in the view field, the automatic focusing system of the infrared cameras has to be able to lock on the target such that the target is kept in focus while the target is moving. In such a dynamic focusing adjustment, it becomes difficult to keep the target properly focused particularly when the target is moving fast. In such a situation, therefore, it is necessary to predict the movement of the target such that the focusing is achieved at the predicted position of the target. Conventionally, there is no known procedure for achieving such a predictive focusing control in the art of infrared cameras.